Alteration in elemental and functional composition of heated peat humic acids

The article analyzes the effect of thermal modification of different-type peat on the alteration of elemental and functional composition of peat humic acids. Based on the data of IR-spectra and readings of electron paramagnetic resonance, structural alterations are identified. It is shown that the impact of peat characteristics on humic acids is preserved after thermal modification. It is revealed that the strongest alteration of humic acid composition and properties caused by peat heating are typical to humic acid samples extracted from the peat with low decomposition degree.


Introduction
Humic acids (HAs) are polydisperse components of soils, peat, and brown coal which are unique as they are characterized by a number of specific features.They are commonly used in the agricultural, environmental, and industrial sectors.
The latest studies have shown that HAs are heterpolycondensates carrying a great variety of functional groups which make macromolecules of HA different in composition, which, in its turn, contributes to the diversity of HA samples.This does not permit a holistic description of HA chemical structure.By now, it has been possible to propose some models and approximate formulas of HA molecule chemical structure [1].
In order to obtain more detailed description of HA chemical structure, it is proposed to use the methods of thermal modification, which makes it possible to reduce the effect of unstable peripheral structural groups on HA properties.
Thermal peat treatment at 250 °С in its decomposing gases is one of the abovementioned methods which is actively employed in the experiment to yield required humic acids from peat [2,3].
The main objective of the study is to identify the effect of thermal modification of different-type peat on the structural alteration of HA.

Research technique
For the research, humic acids extracted from 0.25 mm fraction of air-dried terrestrial, transition, and fen peat samples heated in decomposing gases according to technique [3] have been chosen.The peat samples are characterized by low and medium decomposition degree (R=5…45 %), the data being provided in table 1. ) * W a -moisture of analysis sample, Ad -ash content on air-dried basis, Vоdaf -yield of volatile matter on dry ash-free basis, **daf -on air-dried and ash free basis; О+S is obtained by difference, with S content being not more than 2 % The humic acids (НА и НА 250 ) were extracted in accordance with the following technique [4]: 1) bitumen was extracted by the use of benzene; 2) water-soluble and easyhydrolysable substances were extracted from bitumen-free peat samples (extraction of 4%-HCℓ); 3) humic acids were extracted from peat samples free from bitumen, water-soluble and easy-hydrolysable substances (treatment of 0,1Н by solution NaOH and further precipitation of HA by 10 %-НСℓ).
The functional acid groups (carboxy group and phenolic hydroxyl) in macromolecule HA and НА 250 were identified by defining their total content by means of Barium oxydatum, followed by identification of СООН group by Сalcium acetate, and ОН group identification by the difference (ОН+СООН) -(СООН) according to technique [4].
All samples НА and НА 250 were examined for С, Н, N, O+S content (hereinafter oxygen) by means of automatic elemental analyzer EURO ЕА3000 in accordance with technique [5].The discordance between the data met the following conditions: n ≥ 3, when Р = 0.95.
The conditions of the experiment aimed at identifying paramagnetic properties of НА and НА 250 (readings of electron paramagnetic resonance) were as follows: electron paramagnetic resonance (EPR) -spectrometer Bruker ELEXSYS E-540 (Novosibirsk Institute of Organic Chemistry of the Siberian Branch of Russian Academy of Science); Xband at 10 GHz; g-factor was identified by inside frequency meter spectra, with gauge correction to g-factor DPPH; measurements were carried out on a Bruker ELEXSYS E-540 in EPR ampoules, MW power = 10 dB.The number of electron spins was calculated in accordance with the method developed by «Bruker» -Absolute spins [7].The accuracy of spin number determination was r15 %.
The optical properties of НА and НА 250 were studied using the Nicolet iS10 FT-IR spectrometer, Termo Fisher Scientific.The IR spectrometer was fitted with an add-on device containing ZnSe crystal for frustrated internal reflectance.The experiment parameters were as follows: resolution -4 cm −1 , the number of sample scans and comparison spectrum -128, scan range was from 4000 up to 650 cm −1 .The spectra were registered and processed using Omnic 8.3 [6].

Results and discussion
The obtained data on elemental and functional composition of НА and НА 250 are given in table 2.
The preliminary heat treatment of peat НА 250 led to carbon increase, oxygen and hydrogen decrease.The results demonstrated minimum changes in nitrogen content as compared to alteration degree of other elements.
The above-mentioned regularities are not preserved after peat heat treatment.At the same time, when comparing the relative alteration of element content (+ΔC, -ΔН, -ΔO, +ΔN) in samples НА 250 , the effect of peat R finds its maximum.
Thus, it has been stated that the higher peat R is, the lower is the effect of peat heat treatment on relative alterations of С, Н, О N content in the organic macromolecule of humic acids.
The data on functional composition indicate that the number of oxygen-containing functional groups is within the range typical for peat humic acids in all samples НА and НА 250 [10].
Peat heat treatment leads to the increase in СООН-group content in modified humic acids.The number of hydroxyl ОН-groups decreases or remains unchanged (table 2).
The increase in carboxyl groups in НА 250 composition can stipulate their reaction capacity [11].At the same time, it is necessary to consider not only the number of functional groups, but also their differentiation based on acidity level (difference in molecular environment within the macromolecule).When comparing infrared spectra of НА and НА 250 , it has been revealed that all HA samples are characterized by uniformity of functional groups.
The absorption bands that are typical for HAs are observed in infrared spectra of НА and НА 250 in the ОН-group stretch (3400 см −1 ).The bands are linked by hydrogen bonds.The absorption bands are also observed in СН 2 -group stretch, methyl СН 3 -group (2920 cm − 1 and 2850 cm −1 ), carboxyl С=О-group (1725-1700 cm −1 ), С=С polycoupled aromatic systems and monoaromatic structures (1620-1600 cm −1 and 1520-1500 cm −1 ).Faint absorption bands are defined in vibration bending of С-СН 3 -groups (1390-1370 cm −1 ) and С-О stretch of acids and phenols (1250-1200 cm −1 ).The band 1100-1000 cm −1 corresponds to С-О-group stretch of alcohols (figure 1 Peat heat treatment in НА 250 IR-spectra decreases the intensity of OH-groups and C-Hgroups stretch at 3400 cm −1 and 2920 cm −1 , which, in its turn, indicates the presence of dehydration reactions at low-temperature thermal decomposition at the stage of peat heating. The intensity alteration of absorption bands of С=О-groups at 1725-1700 cm −1 and С-О-groups of acids and phenols at 1250-1200 cm −1 has been determined.This can be caused by interaction of HA thermal decomposition products (alcohol ester, alcohol oxide, and ketone) with water forming new carboxyl groups [10,11].Besides, carboxyl groups can also be formed due to hydrolyze ester bonds that bind other peat components.This fact has been proved by analyzing kinetic parameters of peat components thermal decomposition in [12].
To obtain the data on HA structure, characterized by hetero-polycondensates of irregular composition (methyl-bearing hydroxycarboxylic acids with fused rings including О, N, S-bearing heterocycles and side chains, as well as other functional groups in nucleus and peripheral parts [1,4,10]), atomic ratios Н:С and О:С have been calculated on the basis of elemental composition.In order to obtain the characteristics of НА and НА 250 aromaticity using the Van-Krevelen's equation [11], aromaticity index (А) and the number of aromatic rings (В) for the statistical carbon nucleus of a molecule have been defined.
The obtained number of spins in the sample s = ½ was recalculated for 1g of the sample size (I•10 17 ).The results are given in table 3. The evidence presented suggests that the values of Н:С and О:С atomic ratios for НА/НА 250 (humic acids of air-dried peat/humic acids of heat treated peat) are within the range 0.91 ÷1.25/ 0.81÷1.14 and 0.37 ÷ 0.48/0.33÷0.39.
The alteration of Н:С and О:С values is the evidence of change in the content of macromolecule cyclic components and oxidation degree.The obtained results show that preliminary peat heat treatment decreases the values of Н:С and О:С atomic ratios for НА/НА 250 .This fact indicates that the content of cyclic components can increase in macromolecule.Moreover, it presents the evidence of higher modification degree of modified НА 250 initial substance compared with HA samples.This is proved by the parameters which were obtained using the Van-Krevelen's equation: higher values of А, В were obtained for НА 250 samples compared with HA.
The impact of peat decomposition degree (R) on the calculated atomic relations and the Van-Krevelen's equation parameters is defined: R growth leads to the decrease in Н:С and О:С atomic ratios and increase in aromaticity index and the number of aromatic rings of the statistical carbon nucleus of НА and НА 250 macromolecule.The atomic ratios Н:С and the value of the Van-Krevelen's equation parameters reveal the difference between peripheral and nucleus parts of НА and НА 250 macromolecules.At the same time, it should be noted that modified HAs demonstrate aromaticity increase under the action of preliminary heat treatment which stipulates chemical transformations in peat when it is heated up to 250 °С: 1) organic matter decomposition and emission of gaseous products (СО, СО 2 , Н 2 О) during thermal decomposition; 2) interaction between peat group components and/or their residues in gaseous decomposition environment; 3) synthesis of new HAs [3,6,[10][11][12].
The above is proved by the data obtained using EPR-spectrometer.The data show that peat heat treatment increases EPR signal for НА 250 samples.The concentration of paramagnetic centers also increases: I•10 17 = 0.90÷2.18(НА) and I•10 17 = 1.53÷3.57(НА 250 ).According to [14], such results can be explained by the increase in the degree of aromatic polyconjugation of HA initial matter due to formation of new additional systems of polyconjugation caused by condensation reactions.This fact is in accord with carbon content in НА 250 elemental composition and increasing number of active acidic СООНgroups, which, in its turn, contributes to forming additional paramagnetic centers (presence of hydrogen bonds formed by functional groups) and growing the Van-Krevelen's equation parameters.
The peat heat treatment does not lead to significant change in g-factor.For all examined НА and НА 250 samples, g-factor ranges within 2.0030÷2.0039and approximates the value of unbound electron g-factor (2.0023).

Conclusion
1. Heat treatment of different-type peat increases the carbon content and de-creases the content of hydrogen and oxygen in the elemental composition of HAs. 2. The more peat decomposition degree is, the more indifferent peat is to relative alteration of element content С, Н, О N in organic part of modified HA macromolecule.3. The absorption bands typical for HAs are preserved in IR-spectra of modified HAs. 4. As the result of peat heat treatment, IR-spectra of modified HA demonstrate the decrease in the intensity of OH-groups and C-H-groups stretch at 3400 cm −1 and 2920 cm −1 and increase in the intensity of absorption bands of С=О-groups at 1725-1700 cm −1 and С-О-groups of acids and phenols at 1250-1200 cm −1 .This is the evidence of dehydration reaction and further interaction of pyrogenic water with the products of peat thermal decomposition accompanied by the formation of new carboxyl groups.5. Peat heat treatment increases aromaticity and condensity of HA macro-molecule nucleus part.6. Peat heat treatment also contributes to the concentration of paramagnetic centers.Thus, it can be stated that preliminary peat heat treatment affects the alteration of HA composition and increases the stable nucleus part of HA macromolecule.The preceding observations lead to the conclusion that peat thermal modification has a positive impact on HAs, as it allows eliminating peripheral structures and obtaining new properties.

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The characteristics of HA elemental and functional composition.